FIG. 1 shows schematically a transmission channel of a mobile telephone. The messages are sent in the form of a data stream to a modulation encoding circuit 1 that transforms this data stream into two modulating streams I and Q that are representative of the constellation. Two digital/analog converters 2 and 3 transform the digital signals I and Q into analog signals. Two low-pass filters 4 and 5 filter the signals coming from the digital/analog converters 2 and 3 and deliver them to voltage-to-currrent converters 6 and 7. Two mixers 8 and 9 mix each of the modulating signals that are representative of the streams I and Q by means of a high-frequency carrier. Each mixer 8 or 9 receives the high-frequency carrier in quadrature, in other words the high-frequency signal is phase-shifted by a quarter period between the mixers 8 and 9. An adder circuit 10 then adds together the signals coming from the mixers 8 and 9 and delivers them to a variable-gain amplifier 11. A bandpass filter circuit 12 eliminates the parasitic images created by the mixers 8 and 9, then a power amplifier 13 amplifies the signal and delivers it to an antenna 14.
Mobile telephones have the particular constraint of requiring that the circuits be as small as possible in order to better facilitate integration and also to enable a maximum reduction in power consumption. For this purpose, a conventional approach consisting in partitioning the system into circuits corresponding to the functional partitioning shown in FIG. 1 requires, on the one hand, that the circuit dimensions be large and, on the other, a relatively high power consumption since each stage is powered independently. In order to reduce the size and power consumption of the circuits, a known solution is to produce certain functional elements grouped together within the same circuit stage.
By way of example, the U.S. patent application Ser. No. 2003/0169090 discloses a device that groups the mixer function and amplifier function together on the same stage whose teachings is hereby incorporated by reference in its entirety. However, the circuit disclosed by this US Application requires compensation means whose implementation is relatively complex.
Accordingly, what is needed is a method and system to overcome the problems encountered in the prior art and to provide a circuit that allows a high level of integration and a low power consumption for digital transmission circuits such as are shown in FIG. 1.